The interior surface of long hollow substrates (30-40 ft in length) such as those used in the oil or gas industry may employ various coatings for corrosion, erosion, abrasion and sliding wear protection. In addition, other functionality such as good hydrophobicity (water contact angle, WCA>=90°, super hydrophobicity (WCA˜150°), or ice-phobicity may be useful. Using the current technology plasma immersion ion processing (PIIP) or plasma immersion ion deposition (PhD), hollow substrates up to 20 ft and 4 inches in diameter, or having up to a 60:1 length to diameter ratio, may be deposited with diamond like coating (DLC) or other Si-containing and F-containing hydrocarbon coatings.
However, in processing hollow substrates having a length to diameter ratio of greater than 60:1 for hollow substrate diameters 3 inches or greater, or length to diameter ratios of 48:1 for hollow substrate diameters under 3 inches, uneven plasma distribution occurs inside the hollow substrate as to cause the coating to be so uneven that the center regions typically remain uncoated. This may be because electrons generated inside a hollow substrate biased at a negative potential have to travel out to the vacuum chamber wall (ground) to complete the current flow loop while ions generated from the electron-neutral impact ionization have to go to the hollow substrate wall to complete the current flow loop. The electrons generated near the ends can make it out of the hollow substrate while the electrons generated in deeper regions near the center of the hollow substrate may not travel too far because the electric field is much weaker and the electrons lose the energy nearly completely due to the electron-neutral molecule collisions. As a result, the density of the plasma may be much stronger near the hollow substrate ends and approach zero near the center of the hollow substrate. The result may be that little to no film may be deposited approaching the center of the hollow substrate. Furthermore, coating uniformity may also be affected by the placement and number of pumping stations affixed to a hollow substrate, gas flow rate, pumping speed, etc.
In addition, center electrodes may be used in hollow substrates to increase plasma density uniformity, but not without reported problems. For example, the center electrodes currently sag. In addition, when depositing films including carbon, the center electrode reportedly are coated with a sooty carbon coating, which will typically have adverse effects, such as an increase in the resistivity of the electrode. Loose carbon particles may also randomly fall from the center electrode onto the hollow substrate, which will cause arcing inside the hollow substrate creating holes in the deposited coatings or blocking further deposition causing uneven coatings.